Nothing wrong with sitting per se if he's strong enough to do it unsupported, but Tummy Time will lead more reliably to crawling: he'll need to use his neck muscles to keep the head at the right height, which has the added benefit of getting his vision to converge properly (this is prime developmental time for that!), plus he'll be using his core and upper body more to strnegthen them for crawling (and just overall).
To help him get the hang of the cross-pattern crawl, put him down with a toy JUUUUUST out of reach and see if he can scoot himself toward it. If not, place a hand behind one of his feet and see if he can get the idea to push off that to propel himself forward. Once he gets that consistently, odds are he'll learn to dig his toes for a belly crawl/army crawl, and eventually to a belly-off-the-floor regular cross-pattern (right hand and left knee forward together, then switch).